Better comments for void Item::top_level_item()

    Any new item which can be NULL must implement this call.

  */

  virtual bool is_null() { return 0; }

  /*

    it is "top level" item of WHERE clause and we do not need correct NULL

    handling

    Inform the item that there will be no distinction between its result

    being FALSE or NULL.

    NOTE

      This function will be called for eg. Items that are top-level AND-parts

      of the WHERE clause. Items implementing this function (currently

      Item_cond_and and subquery-related item) enable special optimizations

      when they are "top level".

  */

  virtual void top_level_item() {}

  /*